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Lime mortar or torching is a masonry mortar composed of lime and an aggregate such as sand, mixed with water. It is one of the oldest known types of mortar, used in ancient Rome and ancient Greece, when it largely replaced the clay and gypsum mortars common to construction.
With the introduction of Portland cement during the 19th century, the use of lime mortar in new constructions gradually declined. This was largely due to the ease of use of Portland cement, its quick setting, and high compressive strength. However, the soft and porous properties of lime mortar provide certain advantages when working with softer building materials such as natural stone and terracotta. For this reason, while Portland cement continues to be commonly used in new brick and concrete construction, its use is not recommended in the repair and restoration of brick and stone-built structures originally built using lime mortar. US Park Service Preservation Brief 2
Despite its enduring utility over many centuries (Roman concrete), lime mortar's effectiveness as a building material has not been well understood; time-honoured practices were based on tradition, folklore and trade knowledge, vindicated by the vast number of old buildings that remain standing. Empirical testing in the late 20th century provided a scientific understanding of its remarkable durability. Peter Ellis, The Analysis of Mortar: The Past 20 Years, 2002 Both professionals and do-it-yourself home owners can purchase lime putty mortar (and have their historical mortar matched for both color and content) by companies that specialize in historical preservation and sell pre-mixed mortar in small batches.
The Roman Empire used lime-based mortars extensively. Vitruvius, a Roman architect, provided basic guidelines for lime mortar mixes. The Romans created hydraulic mortars that contained lime and a pozzolan such as brick dust or volcanic ash. These mortars were intended to be used in applications where the presence of water would otherwise not allow the mortar to harden (carbonate) properly.
The slaking process involved in creating a lime putty is an exothermic reaction which initially creates a liquid of a creamy consistency. This is then matured for 2 to 3 months—depending upon environmental conditions—to allow time for it to condense and mature into a lime putty.
A matured lime putty is thixotropic, meaning that when a lime putty is agitated it changes from a putty into a more liquid state. This aids its use for mortars as it makes a mortar easier to work with. If left to stand following agitation a lime putty will slowly revert from a thick liquid to a putty state.
As well as calcium-based limestone, dolomitic limes can be produced which are based on calcium magnesium carbonate.
A frequent source of confusion regarding lime mortar stems from the similarity of the terms hydraulic and hydrated.
If the quicklime is slaked with an excess of water then putty or slurry is produced. If just the right quantity of water is used, the result is a dry material (any excess water escaping as steam during heating). This is ground to make hydrated lime powder.
Hydrated, non-hydraulic lime powder can be mixed with water to form lime putty. Before use putty is usually left in the absence of carbon dioxide (usually under water) to mature. Putty can be matured for as little as 24 hours or for many years; an increased maturation time improves the quality of the putty. There is an argument that a lime putty which has been matured for an extended period (over 12 months) becomes so stiff that it is difficult to work.
There is some dispute (Roman concrete) as to the comparative quality of putty formed from dry hydrated lime compared with that produced as putty at the time of slaking. It is generally agreed that the latter is preferable. A hydrated lime will produce a material which is not as "fatty”, being a common trade term for compounds have a smoother buttery texture when worked. Often, due to lengthy and poor storage, the resulting lime produced by hydrated lime will exhibit longer carbonatation periods as well as lower compressive strengths.
Non-hydraulic lime takes longer to set and is weaker than hydraulic lime, and should not be allowed to freeze before it is well set. Although the setting process can be slow, the drying time of a lime mortar must be regulated at a slow rate to ensure a good final set. A rapidly dried lime mortar will result in a low-strength, poor-quality final mortar often displaying shrinkage cracks. In practice, lime mortars are often protected from direct sunlight and wind with damp Hessian fabric sheeting or sprayed with water to control the drying rates. But it also has the quality of autogenous healing (self healing) where some free lime dissolves in water and is redeposited in any tiny cracks which form.
Mortars using oyster shells can sometimes be identified by the presence of small bits of shell in the exposed mortar joint. In restoration masonry, the bits of shell are sometimes exaggerated to give the viewer the impression of authenticity. Unfortunately, these modern attempts often contain higher than necessary ratios of Portland cement. This can cause failures in the brick if the mortar joint is stronger than the brick elements.
When a stronger lime mortar is required, such as for external or structural purposes, a pozzolan can be added, which improves its compressive strength and helps to protect it from weathering damage. Pozzolans include powdered brick, heat treated clay, silica fume, fly ash, and volcanic materials. The chemical set imparted ranges from very weak to almost as strong as Portland cement.
This can also assist in creating more regulated setting times of the mortar as the pozzolan will create a hydraulic set, which can be of benefit in restoration projects when time scales and ultimately costs need to be monitored and maintained.
Hydraulic lime can be considered, in terms both of properties and manufacture, as part-way between non-hydraulic lime and Portland cement. The limestone used contains sufficient quantities of clay and/or silica. The resultant product will contain Belite but unlike Portland cement not Alite.
It is slaked enough to convert the calcium oxide to calcium hydroxide but not with sufficient water to react with the dicalcium silicate. It is this dicalcium silicate which in combination with water provides the setting properties of hydraulic lime.
Aluminium and magnesium also produce a hydraulic set, and some pozzolans contain these elements.
There are three strength grades for natural hydraulic lime, laid down in the European Norm EN459; NHL2, NHL3.5 and NHL5. The numbers stand for the minimum compressive strength at 28 days in newtons per square millimeter (N/mm2). For example, the NHL 3.5 strength ranges from 3.5 N/mm2 (510 psi) to 10 N/mm2 (1,450 psi). "The Use og Lime-based Mortars in New Build" These are similar to the old classification of feebly hydraulic, moderately hydraulic and eminently hydraulic, and although different, some people continue to refer to them interchangeably. The terminology for hydraulic lime mortars was improved by the skilled French civil engineer Louis Vicat in the 1830s from the older system of water limes and feebly, moderately and eminently. Vicat published his work following research of the use of lime mortars whilst building bridges and roads in his work. The French company Vicat still currently produce natural cements and lime mortars.[5] Names of lime mortars were so varied and conflicting across the European continent that the reclassification has greatly improved the understanding and use of lime mortars.
If shrinkage and cracking of the lime mortar does occur this can be as a result of either
A common method for mixing lime mortar with powdered lime is as follows:
Usually any dampness in the wall will cause the lime mortar to change colour, indicating the presence of moisture. The effect will create an often mottled appearance of a limewashed wall. As the moisture levels within a wall alter, so will the shade of a limewash. The darker the shade of limewash, the more pronounced this effect will become.
A load of mixed lime mortar may be allowed to sit as a lump for some time, without it drying out (it may get a thin crust). When ready to use, this lump may be remixed ('knocked up') again and then used. Traditionally on building sites, prior to the use of mechanical mixers, the lime putty (slaked on site in a pit) was mixed with sand by a labourer who would "beat and ram" the mix with a "larry" (a wide hoe with large holes). This was then covered with sand and allowed to sit for a while (from days to weeks) - a process known as "banking". This lump was then remixed and used as necessary. This process cannot be done with Portland cement.
For preservation purposes, Type N and Type O mortars are often used. A Type N mortar is 1 part Portland, 1 part Lime and 6 parts sand or other aggregate (1:1:6). A Type O mortar is 1 part Portland, 2 parts Lime and 9 parts sand or other aggregate (1:2:9). Straight lime mortar has no Portland, and 1 part Lime to 3 parts sand or other aggregate. The addition of cement or other Pozzolana cement to decrease cure times is referred to as “gauging”. Other than Portland, ash and brick dust have been used to gauge mortars.
For historic restoration purposes, and restoration work involving repointing or brick replacement, masons must discover the original brick and mortar and repair it with a similar material. The National Park Service provides guidance for proper masonry repointing through Preservation Brief 2. In general, Brief 2 suggests that repointing should be done with a similar or weaker mortar. Therefore, a straight lime mortar joint should be repointed in kind. Due to the popularity of Portland cement, this often is not the case. A wall system needs a balance between the mortar and brick that allows the mortar to be the weak part of the unit.
When mortar is stronger than the brick, it prevents any natural movement in the wall, and the faces of the brick will begin to deteriorate. This is a process known as , in which the outer face of a brick degrades and can flake off or turn to powder. There is also a natural movement of water through a masonry wall. A strong Portland cement mix will prevent a free flow of water from a moist to dry area. This can cause rising damp to be trapped within the wall and create system failures. If moisture cannot evaporate into the air, it will accumulate and cause damage to a wall structure. Water freezing in the wall is another cause of spalling.
In restoration work of pre-20th century structures, there should be a high ratio of lime and aggregate to Portland. This reduces the compressive strength of the mortar but allows the wall system to function better. The lime mortar acts as a wick that helps to pull water from the brick. This can help to prevent the older brick from spalling. Even when the brick is a modern, harder element, repointing with a higher ratio lime mortar may help to reduce rising damp.
It may not be advisable for all consumers to use a straight lime mortar. With no Portland in the mix, there is less control over the setting of the mortar. In some cases, a freeze thaw cycle will be enough to create failure in the mortar joint. Straight lime mortar can also take a long time to fully cure and therefore work needs to be performed at a time of year where the weather conditions are conducive to the mortar setting properly. Those conditions are not only above freezing temperatures but also drier seasons. To protect the slow curing mortar from damp, a siloxane can be added to the surface. With historic structures, this may be a controversial strategy as it could have a detrimental effect to the historic fabric.
The presence of Portland allows for a more stable mortar. The stability and predictability make the mixed mortar more user friendly, particularly in applications where entire wall sections are being laid. Contractors and designers may prefer mixes that contain Portland due to the increased compressive strength over a straight lime mortar. As many pre-Portland mix buildings are still standing and have original mortar, the arguments for greater compressive strength and ease of use may be more a result of current practice and a lack of understanding of older techniques.
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